A radio receiver must be able to handle both weak and strong signals. Most often the amplitude detector in the final stage in the receiver must have its input signal at a specific amplitude for optimal performance. This problem is often solved according to prior art by inserting amplifiers having Automatic Gain Control, AGC, functionality into the signal path through the radio receiver. The AGC will change the gain of the amplifier when the strength, i.e. the level, of the input signal changes to keep the detector signal at a predetermined level. The gain change can be achieved in a continuous manner or in discrete steps.
Existing technologies for implementing AGC functions with discrete steps uses switches in some form to realize programmable gain amplifiers, PGA, or programmable attenuators. The switches are usually implemented in FET or MOS technologies or with PIN diodes. The switches can be used to switch in attenuator steps in the signal path, switch in more or less feedback in an amplifier or switching on and off whole amplifier stages. All those methods change the total gain in the receiver.
US patent publications U.S. No. 2009/0079501, denoted ref [1], and U.S. No. 2009/0108931, denoted ref [2], describe circuits with programmable gain implemented with switches.
Ref [1] teaches different automatic gain control designs. Further, a method to compensate the on resistance problem by having “dummy” switches that should track the ones in the feedback. This shows that on resistance is a problem when using switches.
Also Ref [2] teaches automatic gain control comprising switches for setting the attenuation level.
The use of switches for implementing AGC functions has several disadvantages, especially if the radio receiver is implemented at an integrated circuit, e.g. an ASIC, Application Specific Integrated Circuit.
Switches may be implemented in an ASIC by means of Positive Intrinsic Negative, PIN, diode technology, but said diodes are known for consuming a lot of current in some design conditions, and they are therefore not power efficient. When using PIN diodes as switching elements means they are either reversed biased, which will not consume any current, or forward biased, which will consume current. The current is usually large since the trans-conductance, gm, of the diode must be large, which is defined by the diode equation. The trans-conductance must be large to make the PIN-switch having a low on resistance. The following equations shows the relationship between trans-conductance and bias current ID.
            I      D        =                            I          S                ·                  (                                    ⅇ                                                qV                  D                                /                                  (                  kT                  )                                                      -            1                    )                    ≈                        I          S                ·                  ⅇ                                    qV              D                        /                          (              kT              )                                                      g      m        =                            ⅆ                      I            D                                    ⅆ                      V            D                              ≈                        I          D                          kT          /          q                    
The use of Field Effect Transistor, FET, or Metal Oxide Semiconductor, MOS, technology for implementing the switches in the signal path of a receiver chain involves a trade-off between low resistance in on state and low capacitance in off state. Both FET and MOS transistors works with a channel. When they are used as switches the channel is completely open or closed.
To minimize the on resistance and parasitic capacitance the length of the transistor is made as small as the technology allows. Making the switch wide will lower the on resistance but will also increase the parasitic capacitance that will limit the high frequency performance.
FET and MOS switches have a maximum voltage they can handle. Often large signal amplitudes need to be handled. This can limit the usage of a FET or MOS switches.
As described, the use of amplifier circuits having automatic gain control functionality in the signal path of a receiver chain may introduce a number of problems to be solved or avoided.